It is often desirable to provide an ignition advance of some type for the ignition system of a spark-ignited internal combustion engine. For example, an engine speed increases, it is necessary to ignite the gasoline-air mixture sooner, since piston speed increases with engine speed, and it is desirable to have combustion well established by the time a piston of such an engine completes its compression stroke and begins its power stroke. Therefore, as engine speed is increased, additional time is allowed between the ignition of the mixture and transition of the piston from compression to power stroke, known as top-dead center (TDC). Conversely, for ease in starting an engine, it is desirable that there be no ignition of the mixture during the compression stroke to oppose piston motion, so an ignition pulse should be provided either very shortly before, or shortly after, a piston reaches top-dead-center when starting, and also when running at low speed. Such an engine may also be provided with a vacuum advance, sensing vacuum in the intake manifold of an engine as an approximation of the load opposed on a engine, with lowered manifold vacuum being indicative of increased engine load. Under this condition, an advanced ignition pulse should also be provided, so that the charge in the cylinder will be substantially fully ignited when it reaches top-dead center, to produce maximum power. In conventional automobiles and the like, ignition advance based on engine speed may be provided by a centrifugal weight and spring arrangement, operating pins or cams to rotate a plate on which an element, such as breaker points, in mounted, to sense the angular position of a rotating element of the engine. Advance based on vacuum may be accomplished by a vacuum diaphragm rotating a plate on which a centrifugal weight and spring assembly is mounted, so that the vacuum and speed advance mechanisms are additive. Alternatively, many ignition systems for such engines having an electronic, breakerless ignition system may provide an advance electronically, as a function of engine speed, while having a device without moving parts sensing angular position of a rotating element of the engine, mounted on a plate which is rotated by the vacuum diaphragm. In such a case, the vacuum and electronic advances are additive.
It is also desirable to provide an ignition advance for smaller engines, such as those that may be used in vehicles such as snowmobiles, all terrain vehicles, outboard motors for boats, motors for chain saws, and other such portable or mobile application. In the past, many such smaller motors have not been provided with an ignition advance, for the reason that they consume only a small amount of fuel, or do not require optimum power output or efficiency, or do not require frequent changes in engine speed. In such a case, ignition timing is selected at a compromised value, so that the engine will start relatively easily, and will run well at a particular speed, and operate adequately at other rotational speeds. Due to increased fuel costs, it is desirable for such engines to provide a rugged advance mechanism, not subject to contamination from an adverse environment, which can be constructed of components which will remain functional at elevated temperatures, and which are resistant to vibration encountered in such applications. Since engines used for mobile and portable uses must be light and compact, it is necessary to provide an ignition system which can be mounted on, or in close proximity to the heat and vibration environment found in such a location.
Also, in an engine for an automobile or the like, there are differing ignition advance requirements for control of pollution caused by engine exhaust, and for maximum engine efficiency. The products of the combustion process vary with the pressure at which it takes place, which are related partly to the temperature. It is known that combustion should be less complete, when a piston reaches its highest compression point, top-dead center, for minimum exhaust pollution, than for greatest efficiency or power output. It is desirable to have an ignition system which is adapted to minimize exhaust pollutants under some operating condition, and maximum efficiency fuel economy under other conditions. For instance, it may be thought desirable to minimize exhaust pollution, with a decreased efficiency, when a vehicle is operating at lower speed, in a city, and not when it is operating at higher speed. As in known, above approximately 40 miles per hour, the load imposed on the engine of a vehicle is predominantly composed on wind resistance, which increases in a non-linear manner with vehicle speed. It is also desirable to provide a vehicle with the smallest possible engine, reducing the weight of the vehicle, and reducing the amount of power needed to merely operate the engine in overcoming its internal inertia and frictions. Therefore, to achieve adequate power output for highway operations of such a vehicle, it may be found desirable to provide a slight increase in ignition advance at higher engine speeds, so that a small engine will be capable of operating a car at highway speeds, with maximum efficiency and minimum usage of fuel.